Electro-optic modulators have been well known in the art for years, but for multi-channel applications they have suffered from several disadvantages. Prior art modulator arrays have usually been formed from single wafers of electrooptically active material onto which surface electrodes have been attached, to form channels which are defined by the electric field lines within the optical wafer. Cross-talk, or interference between channels, has been a problem because electro-optic modulators are vulnerable on at least two levels. Since the channels are not restricted except by the electric field lines, activity in one channel can easily induce electro-optic interference in a nearby channel. This is in addition to usual electrical cross-talk experienced by closely grouped and unshielded electrical contacts. Also, previous electro-optic modulators and light switches have often relied on surface deposited electrodes, which produce electric field lines that are fringed, rather than channeled and directed. Due to the exponential decay of the electric field intensity inside the material, very high voltages may be required to drive the material to produce the desired electro-optic effect.
Electro-optic materials, such as LiNbO.sub.3, can be expensive, and can require driving voltages. Liquid crystal modulators have also been used, but response times for this type are typically very slow, on the order of milliseconds. Also, the electro-optic effect exhibited by a material can be of several different orders, depending on the material. A first order effect, called the Pockels effect, is linear in its response to increase in applied voltage. A second order effect, called the Kerr effect, is quadratic in its response, thus a greater increase in effect can be produced relative to an increase in voltage. This can theoretically allow smaller driving voltages in a primarily Kerr effect material to be applied to produce a comparable electro-optic effect compared to material which produces primarily Pockels effect.
Lead zirconate titanate polycrystalline ceramic which is doped with lanthanum (PLZT) is a relatively inexpensive, optically transparent ceramic which can be made to exhibit either the quadratic Kerr effect or the linear Pockels effect, depending on the composition, and can be formed into wafers easily and used in sol-gel moldings. The concentrate of lanthanum, or "doping", is variable, and can lead to varying characteristics in the material. PLZT that is commercially available is typically made from a "recipe" which produces a very high dielectric constant .kappa.. Very high .kappa. values produce high capacitance values C, which in turn produce high power requirements, as power (P) is proportional to CV.sup.2 /2 where V=voltage. High power consumption in turn generates heat, so that some modulators that require high voltage also may require cooling. If the proportion of lanthanum dopant, or other components, in the material is adjusted, the dielectric constant value and electro-optic constant value, as well as the type of electro-optic effect (Kerr or Pockels), may also be varied, with the result affecting capacitance and power consumption.
Prior art inventions for modulating light in arrays generally suffer from common problems experienced by multi-channel optical and electrical systems in which the channels are not appropriately isolated. As discussed above, interference is easily induced in nearby channels resulting in cross-talk which can distort image clarity and corrupt data transmissions. Additionally, much of the prior art requires high driving voltages that are incompatible with TTL level power supplies.
U.S. Pat. No. 4,746,942 by Moulin shows a wafer of PLZT electro-optic ceramic material with a large number of surface mounted electrodes. This invention suffers from the disadvantage of cross-talk between channels, although there is discussion of attempts to decrease cross-talk by use of large electrodes and increased space of the electro-optic windows. This results in less efficient use of the material. Although typical driving voltages are not given, with larger areas of material, higher applied voltages become necessary to provide the necessary electric field density in the wafer.
U.S. Pat. No. 4,867,543 by Bennion et al. describes a spatial light modulator made of a solid sheet layer of electro-optic material such as PLZT, which has paired surface electrodes. This has the disadvantage of requiring a driving voltage of approximately 20 volts to produce a phase retardation of PI radians. U.S. Pat. No. 4,406,521 by Mir et al. discloses a panel of electro-optic material which uses electrodes to define pixel regions. It speaks of using voltages in the range of 100-200 volts. U.S. Pat. No. 5,033,814 by Brown et al. also shows a single slab of electro-optic material which requires a driving voltage of 150 volts. U.S. Pat. No. 5,528,414 to Oakley discloses a single wafer of Pockels crystal with surface mounted electrodes requiring a 70 volt driving voltage. Besides being obviously incompatible with TTL voltage levels, none of these inventions have any mechanism for confining electric field lines. Also, in general, use of higher driving voltages will generate heat in the electro-optic material, which can mean that a cooling system may be required.
U.S. Pat. No. 5,220,643 by Collings discusses an array of optical modulators which are built into a neural network. These modulators are mostly of liquid crystal type, although use of PLZT is mentioned. U.S. Pat. No. 4,560,994 by Sprague shows a single slab of electro-optic material with an array of electrodes which create fringe electric fields, which are not channeled. Sarraf's U.S. Pat. No. 5,521,748 also discloses a modulator array in which mirror-like devices deflect or deform when electrostatic force is applied. U.S. Pat. No. 4,367,946 to Varner also discusses a light valve array, with one specifically preferred material being PLZT. However, all four of these inventions can be expected to have the same problems of cross-talk, which the present invention is designed to eliminate.
For the foregoing reasons, there is a need for an array of discrete light modulating elements which can operate at TTL voltage levels, and at high speeds, with almost no cross-talk, and which can be used to produce small pixels or which can be grouped together to create larger pixels and large two dimensional panels or sheets.